It is not an advantage to a humans to kill off the elderly. They help their family and/or tribe by relating experience and knowledge they have gained during their life. Competition among humans is between tribes rather than between individual people. Darwin did not argue for human extermination programs. This is a common misunderstanding of the phrase "survival of the fittest".
Anti-Jewish pogroms have a long history in Europe, and the holocaust was the industrialization of the medieval pogrom.